LEDs find a variety of uses including display devices. As is well known, the emission wavelength of LEDs employing a semiconductor depends on the type of the semiconductor and increases in the order of InGaN, AlGaInP, GaAlAs and GaInAsP. Year by year, the luminance of LEDs has been improved, and a high-luminance LED is now employed as an illuminator or a backlight of liquid crystal display devices. However, studies on further improvement of the luminance are underway.
Among such LEDs of high emission efficiency, U.S. Pat. No. 5,008,718 and JP-A HEI 3-270186 disclose LEDs having a double hetero (DH) junction structure as shown in FIG. 14 or 15. One characteristic feature of these LEDs is a structure in which an active layer 102 for emitting light through recombination of electrons and holes is sandwiched by confinement layers 103 and 104 for confining electrons and holes in the active layer 102. The confinement layers, having a band gap wider than that of the active layer, serve as cladding layers that do not absorb emitted light. Further, reference numeral 101 denotes a substrate, 105 a window layer, 106 a backside electrode, 107 a front side electrode, and 107 an n-type block layer.
As is well known, the wavelength of the light that is emitted from an LED having the aforementioned structure is determined by the composition of the active layer. For example, in an LED employing an active layer 102 formed of AlGaInP (e.g., (AlxGa1-x)0.5In0.5P) having a double hetero (DH) junction structure as shown in FIG. 16, the band gap energy (Eg) of the active layer varies in accordance with the value of x; i.e., in accordance with the equation: Eg=1.91+0.61x (eV) (0≦x≦0.6). Therefore, the wavelength of the light emitted from the LED varies from 650 nm to 545 nm in accordance with the band gap energy of the active layer; i.e., the composition of the active layer. As is also known, when the value of x is increased, the wavelength of the light emitted from the LED is shortened and the intensity of the light decreases considerably.
The reason for the decrease in emitted light intensity has been elucidated as follows. The compositional proportion of gallium (Ga) in the active layer is reduced, since the band gap of the active layer is required to increase for emitting short-wavelength light. However, the band gap difference between the active layer and a confinement layer decreases with a decrease in the compositional proportion. Thus, the potential barrier increases upon injection of holes into the active layer, thereby decreasing the hole-injection efficiency. In addition, the barrier level of the cladding layer with respect to the electrons confined in the active layer decreases, leading to poor electron confinement. As a result, occurrence of recombination of electrons and holes becomes less frequent, thereby lowering emission light output.
In this connection, “High Brightness Light Emitting Diode” (pp. 106, 162 and 168(1996), G. B. Stringfellow et al.) describes that forming a cladding layer from AlInP improves the hole injection efficiency and electron confinement effect of an LED.
JP-A HEI 8-321633 discloses an LED in which a portion of the p-type cladding layer that is in contact with the active layer is formed from an undoped layer having a thickness of about 0.005 to about 0.2 μm.
JP-C 3233569 discloses a configuration in which an additional layer is inserted between a p-GaP window layer and a p-AlGaInP layer or between a p-AlGaInP cladding layer and an AlGaInP active layer so as to prevent notches generated as a result of band discontinuity, wherein the additional layer has an intermediate band gap value falling between the band gap values of the layers sandwiching the additional layer, thereby reducing resistance to forward current.
JP-C 3024484 discloses an LED device having a characteristic cladding layer structure. As shown in FIG. 17, the device includes an active layer 102, an n-type cladding layer 103 disposed on the lower side of the active layer and a p-type cladding layer 104 disposed on the upper side of the active layer. The n-type cladding layer 103 includes a first n-type cladding layer 103a adjacent to the active layer and a second n-type cladding layer 103b adjacent to the first cladding layer. The p-type cladding layer 104 includes a first p-type cladding layer 104a adjacent to the active layer and a second p-type cladding layer 104b adjacent to the first p-type cladding layer 104a. The first cladding layer has a carrier concentration lower than that of the second cladding layer and has a thickness smaller than that of the second n-type cladding layer and greater than a thickness for exerting a quantum mechanical tunnel effect. The potential barrier height provided in a valence band between the active layer and the second cladding layer is predetermined at a level higher than the potential barrier height provided in a valence band between the active layer and the first cladding layer. The first and second cladding layers are formed of AlGaInP, and the ratio of In to AlGa in the first cladding layer is predetermined at a level lower than the ratio of In to AlGa in the second cladding layer. Further, reference numeral 109 denotes a buffer layer.
JP-A 2000-312030 discloses another LED device. As shown in FIG. 18, the device has a stacked structure and electrodes attached to the stacked structure. The stacked structure includes an n-type cladding layer 103 formed of an AlGaInP-based compound semiconductor, an active layer 102 formed of an AlGaInP-based compound semiconductor having a composition for attaining a band gap energy lower than that of the n-type cladding layer, a cladding layer 104 formed of a p-type AlGaInP-based compound semiconductor having a composition for attaining a band gap energy higher than that of the active layer, and a p-type window layer 105 formed of GaP. The device further includes, between the p-type cladding layer 104 and the p-type window layer 105, a mediation layer (forward voltage reduction layer) 110 formed of a material having band-gap energy lower than that of the p-type cladding layer.
JP-A HEI 8-293623 discloses a method for producing an LED device having a DH junction structure. As shown in FIG. 19, the device has a DH junction light-emitting layer including, on a semiconductor substrate 101, an n-type cladding layer 103, an active layer 102 and a p-type cladding layer 104 such that excellent characteristics can be attained without inducing low light emission efficiency caused by diffusion of p-type impurities to the non-doped active layer. The method includes sequential formation of the semiconductor layers, with a portion of the p-type cladding layer 104 on the side of the active layer 102 being virtually formed of a non-doped layer 111, and formation of an electrode 107 through a contact layer 113 on a current diffusion layer (window layer) 105.
U.S. Pat. No. 5,008,718 also discloses an LED device of a structure having a GaP window layer provided on the AlGaInP layer. “J. Crys. Growth” (142, pp. 15-20 (1994), J. Lin et al.) discloses a method for producing an LED device as shown in FIG. 20 including a step of stacking the GaP window layer 105 on the AlGaInP layer, wherein the growth is performed at 800° C. or higher so as to prevent generation of crystal faults.
Generally, p-AlGaInP or p-AlInP is known to have a very small electric conductivity. In order to overcome such a small conductivity, a window layer (or a current diffusion layer) is employed in an LED device so as to increase the area of a light-emitting portion such that current is diffused without flowing locally. However, when the window layer has a large resistivity, the voltage required for supplying rating current to the LED device increases. Thus, the window layer is preferably formed from a substance having as small a resistivity as possible.
Enhancing the crystallinity of the window layer (or current diffusion layer) is effective for lowering the specific resistance of the window layer. However, when the window layer is grown at high temperature in order to enhance crystallinity of the layer, the entirety of the LED device undergoes a high-temperature process. Thus, problems occur in a portion of the device other than the window layer, resulting in a failure to produce an LED device of high output intensity.
Conventionally known light-emitting devices which emit light assuming a color ranging from yellowish green to reddish orange (e.g., light-emitting diodes (LEDs) and laser diodes (LDs)) include a light-emitting device incorporating a light-emitting section formed of an AlGaInP mixed crystal layer, as disclosed in JP-A HEI 8-83927, for example.
The light-emitting device disclosed in JP-A HEI 8-83927 has a configuration including a light-emitting section formed of an AlGaInP mixed crystal layer, a transparent, electrically conductive film formed of indium tin oxide which is laminated on the surface of the light-emitting section, and an upper surface electrode formed on the transparent, electrically conductive film. In the light-emitting device having this configuration, current from the upper surface electrode is caused to diffuse, via the transparent, electrically conductive film, to the largest possible area on the surface of the semiconductor.
However, in the aforementioned conventional light-emitting device, sufficient ohmic contact between the transparent, electrically conductive film and the surface of the light-emitting section fails to be attained, leading to an increase in forward voltage and deterioration of lifetime characteristics. In view of the foregoing, for example, JP-A HEI 11-17220 discloses a light-emitting device exhibiting improved ohmic contact.
The light-emitting device disclosed in JP-A HEI 11-17220 has a configuration including a light-emitting section, a window layer formed on the surface of the light-emitting section, a contact layer formed on the window layer, a transparent, electrically conductive film formed of indium tin oxide (electrically conductive, translucent oxide layer) which is laminated on the contact layer, and an upper surface electrode (upper layer electrode) formed on the transparent, electrically conductive film. In the light-emitting device, current from the upper surface electrode is caused to diffuse, via the transparent, electrically conductive film, contact layer and window layer, to the largest possible area on the surface of the light-emitting section.
In the light-emitting device disclosed in JP-A HEI 11-17220, although ohmic contact between the transparent, electrically conductive film and the semiconductor layer is improved, emitted light is absorbed in the contact layer provided on the light-emitting section, and therefore, high-luminance emission fails to be obtained, and emission efficiency is not improved.
In view of the foregoing, the present inventors have proposed a light-emitting device having a configuration including a semiconductor layer and distribution electrodes provided on a portion of the surface of the semiconductor layer. By virtue of this configuration, the electrical resistance between the distribution electrodes and the semiconductor layer becomes lower than that between a transparent, electrically conductive film and the semiconductor layer, and most of driving current supplied from a pad electrode flows, through a path exhibiting lower electrical resistance, successively to the transparent, electrically conductive film, distribution electrodes, and semiconductor layer (light-emitting section). This light-emitting device is disclosed in JP-A 2001-189493.
In the light-emitting device disclosed in JP-A 2001-189493, since light is emitted from portions of the light-emitting section, which portions correspond to portions located around the distribution electrodes, emission of light does not occur in a region directly below the distribution electrode. Therefore, most of the emitted light is not intercepted by the distribution electrode and can be extracted from the upper portion of the light-emitting device, whereby emission efficiency can be improved. Furthermore, the light-emitting device includes no contact layer, and thus the emitted light can be prevented from being absorbed in a contact layer. This prevention of light absorption also contributes to improvement of emission efficiency.
However, in the case of the light-emitting device disclosed in JP-A 2001-189493, although the distribution electrodes are dispersed and have a small area, light emitted in a region directly below each of the electrodes, is intercepted by the electrode when being extracted from the upper portion of the light-emitting device. This interception has been found to be a cause of lowering of emission efficiency.
In view of the foregoing, the present invention contemplates the provision of a light-emitting diode device capable of attaining high luminance at a wavelength falling within a yellow-green band, in which conventional devices have exhibited considerably decreased output intensity, wherein the device is produced by forming a window layer at a process temperature higher than that conventionally employed, thereby providing a window layer having improved electric-conductivity, and by modifying the device structure so as to prevent variation caused by a high-temperature process.
Other objects of the present invention are to provide a light-emitting diode device that enables attainment of good ohmic contact between an electrode and a semiconductor layer, and effective extraction of light emitted from a light-emitting section while preventing interception of the emitted light, whereby emission efficiency is improved, and to provide a method for producing the light-emitting diode device.